1. Field of the Invention
The present invention relates generally to an improved means of mixing, stirring and maintaining solids in suspension in a liquid. More particularly, relating to an apparatus for maintaining in suspension a complex drilling fluid mixture during storage of the drilling fluid within a storage tank.
2. Background of the Invention
In the operations carried out to drill a borehole through a formation a mixture of chemical, water or oil, and solids such as bentonite, barite and drilled formation particulates, referred to as “drilling fluid” or “drilling mud” is circulated down through a drill string, and through a drill bit where the mixture performs a number of important functions. Such functions include cooling the drill bit, lubricating the drill bit, flushing formation cuttings from the face of the drill bit and then transporting the formation cuttings up annulus between the drill string and the wall of the borehole to the surface of the borehole. At the surface, the drilling fluid containing the formation cuttings is directed through a flow line to a separating device, such as a vibrating screening device know as a “shale shaker” where most but not all of the formation cuttings are separated from the drilling fluid. The drilling fluid then enters a settling tank after passing through the “shale shaker” where a portion of the remaining suspended formation cuttings settle to the bottom.
From the settling tank the drilling fluid flows into the first of one or more storage/suction tanks. When it is necessary to add chemicals, water or oil, and desirable solid to the drilling fluid to adjust or maintain the desired properties of the drilling fluid, they are added through the storage/suction tanks. Normally the storage/suction tanks are fitted with agitators consisting of two large flat fan type blades mounted on a vertical heavy pipe or steel shaft at fixed predetermined distances from the bottom of the tank, The blades rotate at a fixed speed and as a results of the action of the blades on the drilling fluid, the fluid first moves vertically up and then with the effect of gravity down in a circular motion which causes the solids in close vicinity of the blades to remain in suspension. However, there is a significant portion of the fluid in the tank that s not affected by the rotation of the blades, particularly long the perimeter of the tank and solids tend to settle and build up there. The usual way of correcting or compensating for this lack of effectiveness in keeping the solids in suspension is to employ manually manipulated guns to inject fluid through a nozzle at a high velocity into the tanks causing the drilling fluid to roll. This method is effective in putting the drilling fluid in motion and putting and keeping the solids in suspension. However, it has the disadvantage of requiring a worker to be stationed at each tank to operate the gun. As a result it is only practical to use this technique intermittently and as a result solids settle to the bottom of the tanks where they are difficult to reintegrate into the drilling fluid.
There are many deficiencies with the current preferred system. First, is that the useful and valuable solids in the drilling fluid settle to the bottom of the tanks along with the formation cuttings and can not be picked up by the pump suction to be properly processed by the drilling fluid “centrifuge” or the “desander” where the high cost drilling fluid and including most of the desirable solids are separated from the undesirable formation cuttings. This results in some of the high cost drilling fluid being discarded along with the formation cuttings during or at the end of the well. Second, the build up of solids in the storage/suction tanks results in a lengthy, difficult manual cleaning of each tank at regular intervals during the drilling of a borehole. Third, more chemicals and more beneficial solids must be used to maintain the properties of the drilling fluid in the optimum range than what would be required if the drilling fluids in the tanks could be mixed and agitated in a manner that would keep all solids, both the beneficial and undesirable solids in suspension.
It should be noted that drilling fluids are very expensive sophisticated substances whose properties such as viscosity, density, shear resistance and stability must be carefully and accurately maintained and controlled. Unwanted formation cutting solids in the fluid make it more difficult to manage the properties.